III-nitride based nanowire light-emitting diodes (LEDs) have received a staggering response as a future candidate for solid-state lighting and full-color displays due to their unique and exceptional features including drastically reduced polarization fields, dislocation densities as well as the associated quantum confined Stark effect (QCSE) on account of their effective strain relaxation. Moab Rajan Philip and his supervisor Dr. Hieu P Nguyen in the Nano-Optoelectronic Materials and Devices Laboratory at the New Jersey Institute of Technology (NJIT) present astonishing ideas to fabricate and control the color emission of III-nitride nanowire LEDs via molecular beam epitaxy (MBE) growth techniques. The advantage of such LEDs and their characteristics is also discussed.

Researchers from the New Jersey Institute of Technology (NJIT) present astonishing ideas to fabricate and control the color emission of III-nitride nanowire LEDs

High brightness LEDs are already penetrating the market and replacing conventional incandescent light sources and being utilized in areas of general lighting, traffic lights, display applications and automotive lighting. The scope and need for color-tunable novel high efficiency LEDs has never been the same. However, engineering these materials has been constrained due to several fundamental limitations, including lattice mismatch and the limitation of bandgap engineered materials. In this regard, III-nitride nanowire structures give flexibility to this problem with the ability to generate high quality tunable color emission without much effort in contrast to the conventional thin-film structures. In this paper, the ability of the III-nitride nanowires LEDs for color tuning via MBE and the fabrication of phosphor-free white LEDs using this comprehensive approach along with the characterization of the fabricated LEDs will be illustrated.

Introduction

GaN-based nanowire heterostructures have been sought out in recent times for their potential to generate emission wavelengths varying from ultraviolet to near-infrared regions, which in turn, gives the lighting industry additional flexibility to develop phosphor-free white-LEDs. The latter mentioned phosphor-free nanowire white-LEDs has been shown to have tremendous potential because of their higher efficiency, longer lifetime, and better light quality in contrast to existing phosphor-based lighting technologies. Phosphor-free white-LEDs have been envisioned in recent times by means of axial or radially aligned bottom-up nanowire heterostructures including dot/disk/well-in-a-wire or core–shell structures. Their device performance, however, is severely constrained because of nonradiative surface recombination. The large surface recombination is reflected from the very short carrier lifetime (0.3 ns) values reported for conventional nanowire structures. The use of AlGaN shell in InGaN/(Al)GaN core–shell LEDs leads to an enhancement in the carrier confinement and reduced nonradiative surface recombination, thus resulting in an overall boosting of the carrier injection into the active region of the LED device. In the afore-mentioned context, a unique core–shell nanowire LED heterostructure was introduced by utilizing self-organized InGaN/AlGaN heterostructure. Such LED structures offers significantly improved carrier injection efficiency and output power, in comparison to the typical InGaN/GaN nanowire LEDs. By altering the size and/or composition of the InGaN active region in the core–shell nanowire LED arrays during the epitaxial growth of nanowire LEDs, the device light emission properties, including the correlated color temperature (CCT) and color rendering index (CRI) can be readily modified. The electroluminescence (EL) spectra show a very broad spectral linewidth and fully cover the entire visible spectrum. The fabricated InGaN/AlGaN LEDs exhibit excellent current–voltage characteristics. The phosphor-free InGaN/AlGaN nanowire white-LEDs display high CRI up to ~98, which is more efficient compared to the current phosphor-based white-LED technologies. Moreover, we have successfully fabricated phosphor-free nanowire LEDs onto metal substrates via a substrate transferring process. Compared to conventional nanowire LEDs on Si, nanowire LEDs on copper (Cu) exhibit several advantages, including more efficient thermal management and enhanced light extraction efficiency due to the usage of metal-reflector and highly thermally conductive metal substrates. The LED on Cu, therefore, has stronger photoluminescence, electroluminescence intensities and better current-voltage characteristics compared to the conventional nanowire LED on Si, paving the way for a whole new generation optoelectronic device for the future solid-state lighting, flexible displays and wearable electronic applications.

Device Structure

The InGaN/AlGaN LED nanowire heterostructures, were spontaneously grown on n-type Si substrates under nitrogen rich conditions by a radiofrequency plasma-assisted nitrogen source equipped Veeco Gen II MBE system. Silicon and magnesium were used as n- and p- type dopants for GaN nanowires, respectively. The LED active region consists of ten vertically aligned InGaN dots, sandwiched between 3 nm AlGaN barrier layers. The nanowire diameter and density can be tuned by varying the substrate temperature and/or In/Ga flux ratios, whereas the nanowire length can be controlled by the growth duration. The 45-degree tilted SEM image (Figure 1(a)) shows uniform nanowires with high areal density of ~1 x 1010cm-2. During the growth process of AlGaN barrier, an AlGaN downward-bending shell layer was also formed around the InGaN layers due to the diffusion-controlled growth process[1, 2]. These core-shell structures exhibit low nonradiative surface recombination, and improved carrier injection efficiency compared to InGaN/GaN nanowire LEDs without using core-shell structures.

Results and Discussion

The emission wavelengths can be readily engineered across the entire visible wavelength regime thus easily generating the full color spectrum. The optical properties of InGaN/AlGaN LED nanowire heterostructures (Figure 1(b)) revealed that peak emission wavelengths of InGaN/AlGaN core–shell nanowire LEDs ranging from 460 to 670 nm can be attained. The effective variation of the In composition in the active region of the InGaN active region was achieved by controlling the growth temperatures and/or In/Ga flux ratios. The longer wavelength like red light is often linked with a lower bandgap material for the active region. Thus, the desired color emission from blue to red color in the visible regime of the electromagnetic spectrum viz towards a higher wavelength was obtained by altering the In composition from a lower value to a higher value. The latter engineered heterostructures are being used to realize multiple color devices as expected.

Furthermore, the core–shell LED samples have an enhanced optical intensity (a factor of >8 times higher of photoluminescence intensity) in contrast to the conventional InGaN/GaN device. This has been linked to the fact that radiative recombination is more assertive on account of the effective carrier confinement in the active region of InGaN/AlGaN LEDs, being more efficient when compared to that of InGaN/GaN LED devoid of AlGaN core–shell structures. This enhancement is associated to the strong carrier confinement, due to the effective lateral confinement offered by the large bandgap AlGaN barrier and drastically reduced nonradiative surface recombination. In addition, the core-shell InGaN/AlGaN LEDs boasts of significantly improved carrier lifetime, which is more than 15 times higher when compared to the conventional InGaN/GaN LEDs. The latter improved carrier lifetime stems from the reduced nonradiative surface recombination arising due to the effective lateral confinement offered by the higher bandgap of AlGaN shell layer in the core-shell LED structure.

Figure 2: Electroluminescence spectra of various InGaN/AlGaN LEDs (a) with distinct emission colors, along with their optical image. Electroluminescence spectra of the phosphor-free core–shell nanowire white-LED (b) under injection current along with the optical image of the white LED[1]

The CRI of phosphor-free white LEDs can be engineered by controlling the In composition in the InGaN active region, thus leading to various emission spectra and different spectral power distribution (SPD) characteristics. In contrast to conventional planar LEDs, InGaN/AlGaN core–shell nanowire LEDs offer a unique feature of providing flexibility in engineering the peak emission wavelengths and spectral linewidths, which in turn are the two most vital criteria for CCT and CRI. The EL spectra with injection currents varying from 50 mA to 250 mA for phosphor-free white LEDs is provided in Figure 3(a) while the Commission Internationale de l’Elcairage (CIE) coordinates for the same LED device is depicted in Figure 3(b). The fabricated white-LED exhibits broad spectra with a fullwidth of half maximum of ~171 nm and offers SPD values in the range of 410–800 nm, respectively. The resulted CRI values of around ~98 were measured for those phosphor-free nanowire LEDs.

III-nitride nanowire LEDs are normally grown on Si substrates, which may largely absorb photon emitted from the LED active region, severely limiting the light output power. Moreover, Si semiconductor generally exhibits low electrical conductivity and thermal expansion coefficients compared to metal substrate. High power LED applications, however, require large-area device size and operate at high injection current which mostly will heat up the devices. The resulted quantum efficiencies, output power and lifetime reduce rapidly when the junction temperature increases. Therefore, managing heat dissipation should be seriously considered. Besides the applications in solid-state lighting illumination, the use of LEDs in telecommunications, and decoration displays for flexible electronics devices has also been intensively developed due to the feasible integration of such LEDs in these electronic devices. In this regard, we have successfully fabricated full-color and phosphor-free white-LEDs on Cu substrates. Nanowire LED structures were first grown on silicon-on-insulator (SOI) substrates by MBE, then were transferred onto Cu substrates via the substrate-transfer process. Compared to conventional nanowire LEDs on Si, nanowire LEDs on Cu exhibit offer more efficient thermal management and enhanced light extraction efficiency due to the usage of metal-reflector and highly thermally conductive metal substrates. The LED on Cu, therefore, has stronger photoluminescence, electroluminescence intensities and better current-voltage characteristics compared to the conventional nanowire LED on Si. The LEDs on Cu substrate exhibit excellent current-voltage characteristics and show lower leakage current and slightly higher current in forward bias, compared to the LED device on Si substrate at the same voltage, shown in Figure 4(a). The optical image of light emission from the LED device on Cu substrate is presented in the inset of Figure 4(a). Illustrated in Figure 4(b), the nanowire LED on Cu shows higher light output power compared to that of nanowire LED on Si[4]. The enhanced output power is attributed to the enhanced light extraction efficiency and better heat management in LED on Cu substrates which was explained previously.

Figure 4: Current−voltage (a) and light output power vs injection current (b) characteristics of the conventional nanowire LED on the Si substrate and nanowire LED on metal Cu substrate[4]

In conclusion, high performance InGaN/AlGaN core-shell nanowire LEDs on Si and metal substrates have been developed. The color properties of such full-color nanowire LEDs can be adjusted by controlling the indium composition in the InGaN/AlGaN active region of the LED structures. This study addressed some major roadblocks for the practical applications of nanowire-based LEDs and has further provided an entirely new avenue for the development of future high efficiency phosphor-free white LEDs as well as full-color LEDs with tunable emission for advanced lighting applications.

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